Absolutely correct. When a round "cooks off" on its own (i.e., not chambered in a firearm), the bullet and the brass will likely be found lying a few inches apart. If you think about the physics, the bullet is the most massive piece of an un-chambered cartridge. When the powder cooks off, the brass will burst and may move (it is light) - the bullet itself will barely move (conservation of momentum). Now, if that cartridge is chambered in a firearm whose receiver will contain the pressure - and whose mass is much greater than the bullet, the bullet will exit as designed with high velocity. In the news story, it sounds as though the homeowner did have loaded guns in the house in addition to the ammo.The whole "ammo cooking off" fear is pretty overblown. A cartridge forcefully ejects a bullet when chambered only because all the pressure is vented in a single direction -- straight down the barrel.
You could drop a handful of cartridges onto a campfire and they would burn, but there is no pressure being released and isn't particularly dangerous.
Incidentally, the show "Mythbusters" conducted a very good demonstration of this a season or two ago.